1. Field of the Invention
The present invention relates to non-impact printing apparatus for recording on a moving photoreceptor or the like and a printhead for use therewith. The invention also relates to an improved driver chip for use with such non-impact printing apparatus.
2. Brief Description of the Prior Art
In the prior art as exemplified by U.S. Pat. No. 4,885,597, printer apparatus is described which comprises a multiplicity of individually addressable and energizable point-like radiation sources, such as LED's, arranged in a row for exposing points upon a photoreceptor during movement thereof relative to and in a direction normal to the row. Driver circuits are provided for simultaneously energizing the radiation sources responsive to respective data bit input signals applied to the driver circuits during an information line period. The print or recording head includes a support upon which are mounted chips placed end to end and upon each of which are located a group of LED's. The driver circuits are formed as integrated circuits and are incorporated in chips that are located to each side of the linear array of LED chips. The driver circuits in this apparatus each include a shift register for serially reading-in data-bit signals and for driving respective LED's in accordance with the data signals.
Associated with each driver chip is a current-level controller that controls the level of current into the LED's of that group during recording. The controller comprises a current mirror having a master control circuit whose current is mirrored in slave circuits to which the LED's are connected. One advantage of this prior art printer apparatus is that current to the LED's may be changed automatically as needed, due to changes in aging or temperature of the printhead. As such changes affect the light output of the LED's, the changes to the current compensate for same so that some uniformity is provided to the recording apparatus.
In the current mirror described in this prior art, there are provided two avenues for adjustability. Firstly, there is a "system bias" voltage which is adjustable to compensate for loss in intensity of light output from the LED's due to aging, i.e., hours of use. Since aging will affect most LED's on a printhead to about the same extent, the loss in intensity due to aging may be overcome by changing the system bias voltage which causes additional current to be provided to the LED's. This change in system bias voltage may be characterized as a "global" change since the change in system bias voltage affects all driver chips on the printhead. In order to change system bias voltage, a new digital word is sent to a digital current mirror control that is separate from the driver chips. By enabling the appropriate current-carrying transistors, a new level of system bias may be provided to each driver chip. Incorporated within each driver chip is an additional current mirror that is also subject to digital regulation and can be used to provide "local" regulation or control for such localized effects as temperature and other chip to chip nonuniformities.
While the above approach can work well, there are occasions where due to processing conditions used in manufacturing the circuit providing system bias voltage and at least some of the driver chips that a change in system bias voltage affects different driver chips on the same printhead to different extents. Also, the long lead lines for distributing system bias voltage to each of the driver chips subjects this voltage to noise, thereby further affecting the driver chips differently.
A further problem with the approach of the prior art is that calibration can be difficult in that where for the same system bias level, the current to each LED may be varied from zero to some large value depending upon the digital word sent to each driver chip controlling the localized part of the current control. In order to accommodate this broad range of possible current levels, fine control can only be obtained by increasing the number of possible levels. However, this adds more complexity to the printhead in that it requires more data bits to provide digital regulation of current. The other alternative of settling for coarse control of allowable changes to current level results in compromise to uniformity control.
It is an object of the invention to improve upon the printer apparatus of the prior art to overcome the above noted problems.